Obtaining a uniform dispersion of elemental sulfur in rubber stocks has been a serious problem for the rubber industry and one which has received a great deal of attention. Problems attributable to poor dispersion of sulfur in the rubber include migration of the sulfur to the surface of the rubber stock commonly referred to as "bloom"; decrease of tack at the surface of the rubber stock; variation of the physical properties of vulcanizates from batch to batch; and others.
Compounds containing high sulfur levels can exhibit problems with sulfur bloom on the surface of the unvulcanized rubber. This surface layer of sulfur crystallizes causing a loss of building tack which can cause problems in tire building.
Numerous modifications of standard rubber processing techniques have been utilized to minimize the sulfur bloom tendencies. These prior methodologies include the use of insoluble sulfur in the compound; limiting the compound mixing temperatures during the sulfur addition stage; and minimizing the heat history that the compound is exposed to during processing.
Insoluble sulfur is formed by rapidly quenching molten sulfur that is above 159.degree. C. (preferably 200.degree. -250.degree. C.). This product consists primarily of long chain sulfur molecules and a lesser amount of soluble S.sub.8 rings. There is a tendency for the long chain molecules to revert to the more stable soluble form if exposed to higher temperatures, long storage times and/or hostile storage environments.
Commercial insoluble sulfur products contain a stabilizer to reduce this tendency. When insoluble sulfur is mixed in a rubber compound, it exists as more or less discreet particles of varying size in the rubber phase. Above about 118.degree. C. sbbstantial reversion to the soluble sulfur form occurs with resulting sulfur bloom.
An approach taken over the years has been to combine sulfur with an unconjugated diene which is believed to enhance the compatibility with the rubber. The polymeric structure is also believed to improve the stability of the sulfur chains against breakdown to soluble S.sub.8 units at normal storage and processing temperatures yet readily allow the release of sulfur for crosslinking at vulcanizing temperatures.
U.K. Patent No. 1,232,056 discloses a method of preparing a vulcanizing agent for natural and synthetic rubbers which comprises heating together at a temperature of from 100.degree.-250.degree. C. between 3 and 50 parts by weight of sulfur and one part by weight of a conjugated diolefin in the presence of a catalytic amount of an amine, such as a dimethyl-substituted tertiary amine.
U.S. Pat. No. 2,989,513 discloses a rubber composition comprising natural or synthetic elastomers and from about 1 to 12% by weight based on the weight of said rubber of a curing agent comprising at least one interpolymer of sulfur and an olefinic hydrocarbon selected from styrene, alpha-methylstyrene, butene, isobutylene, diisobutylene, triisobutylene, ethylene and propylene.
U.S. Pat. No. 3,544,492 discloses an improved curing agent which is a resinous composition formed by the reaction of one or more olefinic materials and a s-triazine, substituted with three groups containing activated terminal unsaturation and sulfur.
U.S. Pat. No. 3,264,239 discloses a process and a vulcanizing agent which comprises preparing a mixture of sulfur, linseed oil and dicyclopentadiene, heating the resulting mixture at 125.degree.-135.degree. C. for at least 5 hours to form an interpolymer, and cooling and isolating the interpolymeric product.
U.S. Pat. No. 3,523,926 discloses a vulcanizing agent for rubbers which is prepared by heating conjugated diolefins with sulfur in the presence of catalytic amounts of amines.
U.S. Pat. No. 4,564,670 describes a disperse sulfur product formed by dispersing particulate sulfur in a liquid poly(cis-isoprene) dispersion agent. The product can be formed by simply mixing the liquid poly(cis-isoprene) dispersion agent with a major amount of sulfur until the desired product results.
Canadian Pat. No. 965,231 claims a method for improving the dispersibility of insoluble sulfur in rubber which comprises admixing insoluble sulfur containing up to about 70% by weight soluble sulfur with from about 0.3 to 5% by weight based on the total weight of the sulfur of a dispersing aid selected from a specific group of alkyl-phenoxypoly(ethyleneoxy)ethanol compounds.
Japanese Publication No. 57-133135 discloses a rubber composition with improved sulfur bloom characterized by the addition of triisopropanolamine, diisopropanolamine, monoisopropanaolamine or blends thereof to a rubber composition composed of 2 to 10 weight parts of sulfur as a vulcanizing agent blended in 100 parts of rubber selected from natural rubber, synthetic rubber or rubber blended from the two.
From a review of the prior art methodologies for preparing sulfur-olefin adducts, it is quite apparent that the reaction product of sulfur and an olefin results or can result in materials that are viscous liquids or solids. For example, U.S. Pat. No. 3,259,598 teaches that a sulfur, linseed oil, styrene reaction product can be used to vulcanize rubber. The product from this reaction mixture must be pulverized before it can be incorporated into the elastomer due to the physical properties of the sulfur-olefin adduct.
Uniform dispersion of the sulfur in the rubber is a prerequisite for uniform vulcanization and vulcanizates with optimum mechanical properties and many sulfur-olefin vulcanizing compositions of the prior art require that grinding or milling steps be performed on the sulfur-olefin adduct prior to its use in rubber. This problem has been overcome through the instant invention wherein the sulfur-olefin adducts are prepared by reacting sulfur and an olefin at 140.degree.-160.degree. C. with agitation in water which optionally contains a base as a catalyst and a dispersing agent. The prior art does not suggest or disclose a process for the preparation of sulfur-olefin adducts which is characterized as heating sulfur, an olefin, water, base and optionally a dispersing agent to 120.degree.-200.degree. C. with agitation, cooling the reaction mixture and filtering the sulfur/olefin adduct beads.
The water serves as a medium in which the sulfur can melt and react with the olefin in a particle form. When the reaction mixture is cooled, the sulfur/olefin adduct is frozen into a bead form. The water also acts as a heat sink for the exothermic reaction.